1. Field of Invention
The invention relates to the fabrication of semiconductors, and more particularly to equipment using chemical mechanical polishing techniques. The invention provides a platen and other components for chemical mechanical polishing apparatus that are resistant to corrosive conditions encountered in the polishing process.
2. Background of the Related Art
In the fabrication of semiconductors, thin wafers of silicon are subject to a series of processes that create a plurality of semiconductors on the wafer surface. Some of these steps may, for example, etch the silicon surface, while others deposit semiconductor component layers using techniques that form thin film layers, like chemical or physical vapor deposition, ion implantation, and the like. At certain intervals in the manufacturing process, the surface of the semiconductor wafer must be polished in order to planarize the surface, and/or to selectively remove certain surface features. Generally, this polishing step is carried out using "chemical mechanical polishing` apparatus and techniques.
In CMP fabrication techniques, a free abrasive chemical slurry is often used along with a rotating polishing pad, linear polishing belt, or rotating drum to contact the workpiece surface and to polish and planarize that surface. Typical examples of these types of apparatus are described in U.S. Pat. No. 5,329,732, assigned to SpeedFam-IPEC disclosing a rotating polishing pad polisher; PCT Publication WO 97/20660, assigned to Applied Materials disclosing a linear belt polisher; and U.S. Pat. No. 5,643,056, assigned to Ebara Corporation and Kabushiki Kaisha Toshiba disclosing a rotating drum polisher. The disclosures of the foregoing patents, in relevant part, are incorporated herein, by reference.
As pointed out above, in chemical mechanical polishing, also known as "CMP", a chemical abrasive slurry is frequently used. The slurry is selected based on its properties, to facilitate the selective removal of the particular materials to be polished from the wafer surface. Thus, while most slurries contain fine abrasive particles such as alumina or silica, other slurry chemistry may vary widely. For example, a slurry may have a pH in the range from about 1 (highly acidic) to about 12-13 (highly alkaline). Certain components of the CMP apparatus are inevitably exposed to this slurry, that is both abrasive and potentially corrosive, depending upon its chemistry.
In addition, as might be expected, a polishing process generates heat due to polishing friction. Ordinarily, parts of the apparatus exposed to this friction-generated heat are sufficiently massive or composed of a material with high specific heat so as to provide a heat sink so that the temperature of the apparatus does not increase significantly. For example, FIG. 1 is an illustrative embodiment of a portion of a CMP apparatus that includes a platen 110, with an outer surface 120, mounted to a platen support structure or water jacket 140, that is fixed or rotating. A polishing pad 150 is mounted to the platen outer surface 120. A semiconductor wafer is secured to a wafer carrier 160, that is located adjacent the polishing pad 150 so that the pad can polish a wafer held in the wafer carrier 160, when the pad is brought into contact with the wafer.
The platen 110 is typically constructed of aluminum, stainless steel, Inconel.RTM., ceramics, and the like. The type of material of construction, and platen design, are constrained by mechanical and thermal considerations. For example, temperature changes must be minimized to reduce dimensional changes that might occur with temperature change. The CMP process must be able to polish wafer surfaces to within fine tolerances, of the order of a few microns. Accordingly, dimensional stability of the platen is desirable and necessary. Further, temperature increase may affect the reactivity of chemicals of the abrasive slurry, and may contribute to undesirable side effects, such as reduced selectivity, or faster than expected polishing rates.
With regard to temperature control, platens may be divided into active and passive systems. In passive systems, there is provided a component with a large thermal inertia, often a material with high specific heat, so that heat generated by the polishing process is absorbed without significant temperature change. On the other hand, active systems remove the frictional heat generated by use of a "chiller", heat exchanger, or other suitable means.
The use of active heat removal systems is preferred, because these systems have lower mass, adjustable controls, better process parameter ranges, and lower costs than passive systems. The most common material for platens in active systems is aluminum, due to its high strength, low mass, and good thermal conductivity. Aluminums high thermal conductivity, coupled with its high thermal expansion coefficient, requires that temperatures be precisely controlled to limit warping of the platen, with resultant deleterious effect on polishing. Aluminum also has the disadvantage that it is amphoteric--i.e., it is susceptible to corrosion by both acidic and basic slurry components. Moreover, aluminum oxidizes readily in the presence of water.
Chemicals contained in the abrasive slurries, and the use of deionized water (in the rinsing) contribute to enhancing corrosion of the materials and components of equipment. Further, microscopic components released from the equipment due to the corrosion process results in contaminated wafers and defective semiconductor devices. If significant corrosion of the equipment results from the action of chemicals, the equipment component must be replaced. This incurs both equipment repair costs as well as loss of equipment use (down time), both of which are undesirable.
In the past, others have tried to address the issue of corrosion through use of a refractory metal oxide coating. For example, U.S. Pat. No. 5,743,788 shows the use of metal oxide compounds. However, these coatings are brittle and pose issues with regard to impact resistance, impurities, porosity, and matching thermal expansion with the underlying substrate. Further, the coatings have a relatively low adhesion strength which, combined with the brittleness and thermal expansion matching issues, result in excessive stress at the interface between the coating layer and the underlying platen substrate. These stresses ultimately result in delamination of the coating from the substrate, or cracking of the coating. Moreover, aluminum is a common platen material and its oxides are only stable in the pH range about 5.5 to about 9. Therefore, aluminum oxide coating is not suitable for acidic or highly alkaline slurries.
Attempts have also been made to use anodizing and polymer coatings (epoxy paints) as coating materials to protect exposed components of the CMP equipment from chemical attack. These have found limited application in CMP.